1. Field of the Invention
This invention relates generally to a method of manufacturing tires that are cured in a mold, and, more specifically, to a method for fabricating a tire or tire tread around a portable mold core, both of which are moved through various tire building stages until the tire or tire tread and core are placed into a mold where the rubber materials of the tire are cured.
2. Description of the Related Art
It is commonly known that tires are made by wrapping a number of tire products including carcass plies, belts and tread rubber sequentially about a building drum or other apparatus that provides a building surface. Many of these products include a rubber or some other thermoset elastomer in them that is in the “green” or uncured state during the tire building process. It is desirable that these materials be cured after the tire has been built so that their molecules cross link, which imparts them with greater durability which is particularly advantageous in promoting the endurance and life of the tire in use. In order to accomplish this, the green tire is typically placed into a mold where sufficient heat and pressure are added to the rubber and elastomer materials so that curing or vulcanization occurs, creating the cross linked molecules that provide the aforementioned desirable characteristics.
Pneumatic tires also have sidewalls that terminate in bead sections, all of which are built onto the building drum, and that provide the means by which the tire is held onto the rim of a vehicle. This type of tire is often cured in a mold whose surfaces that form the outer portions of the tire, including the width of the tire and its outer circumference, which are spatially fixed and that also has an inflatable bladder that mates with the inner surfaces of the tire. As can be imagined, the volume of tires placed into such a mold varies as a result of the tire building process as the tire products are laid onto the building drum. This is attributed to slight mass variations within the products that make up the tire as well as the imprecision with which these products are placed on top of each other. Also, there are usually tiny air pockets that have been trapped during the fabrication of the tire as one layer is laid upon another. Once a green tire is placed into the mold, the inflatable bladder is expanded with enough pressure so that any gaps between the fixed surfaces of the mold and the outer surfaces of the tire as well as any air pockets within the tire itself are removed, helping to ensure that heat and pressure can be effectively transferred to the rubber materials for proper vulcanization. The typical industry practice results in a situation where an uncured tire is often smaller than the volume of the closed mold and that this design criteria accounts for any customary building product volume variation. When the uncured tire is smaller than the closed mold volume, the mold can be closed via conventional tire curing press technology. The inflation bladder expansion accommodates any normal volume variations that occur from uncured tire to uncured tire during production. Further, should conditions result in uncured tires that are oversized as compared to the volume of the mold, the pressure within the inflatable bladder, and its flexibility, allow the mold to close properly, without damage to the mold itself or the surrounding mechanisms. Once the mold is fully closed and locked, the inflatable bladder is then expanded so that proper conduction of heat and pressure from the mold to the tire can be achieved as previously described.
However, such molding technology does not work effectively with all types of tires. For example, some non-pneumatic tires have a shear band in their tread that comprises inextensible membranes that surround a shear layer made of an elastomer material. For an example of this construction, see U.S. Pat. Nos. 6,769,465; 7,013,939 and 7,201,194 that are commonly owned by the assignee of the present application. The membranes are very stiff, i.e. 100 to 1000 times more stiff than rubber, as they often use reinforcements made of uninterrupted steel cord. Further, the orientation of the reinforcements within the membranes is generally circumferential. The combined result of reinforcement and orientation is the inextensible nature of these membranes. This is different from typical pneumatic tire industry practice, where the steel cords are angled with respect to the circumference of the tire belts. The presence of this cord angle introduces interruptions of the cord along the circumferential direction of the belts. These interruptions of the steel cords, along with the low stiffness circumferential reinforcements that are also in typical use by the tire industry, result in uncured pneumatic tires that are extensible in circumference and therefore are compatible with inflatable bladder molding technology.
When inflatable bladders are used with tire constructions that include inextensible membranes, one of two adverse reactions generally occur. When there is insufficient uncured tire volume, the inflatable bladder will force an undesirable flow of rubber between the steel cords, which degrades the mechanical performance of the cured tire. Conversely, when there is excess uncured tire volume, the closing mold will distort the belts as the tire is forced to have a smaller circumference. The inextensible belts must buckle, since they cannot change circumference. The inflatable bladders, lacking any mechanism to constrain distortion, simply accommodate the buckling and thus allow the distortion to become a permanent feature of the cured tire.
Accordingly, it is desirable to find a method for controlling the volume of the tire tread or tire precisely so that it can fit snugly within a mold without damaging the tire tread or tire, or other components thereof, upon mold closing and so that the tread or tire expands sufficiently as the mold heats up so that it contacts the mold without causing the flow of rubber between the steel cords of an inextensible membrane, helping to ensure that enough pressure and heat are transferred to the elastomer materials in the tread or tire for proper vulcanization and the removal of any air pockets found within the tread or tire.